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Background

The Semantic Web Stack

The Semantic Web identifies a set of technologies and standards that form the basic building blocks of an infrastructure that supports the vision of the meaningful Web. Figure 1 illustrates the different parts of the SW architecture. It starts with the foundation of URI (universal resource identifier) and Unicode. URI is a formatted string that serves as a means of identifying abstract or physical resources. For example, http://eden.dei.uc.pt/~jcardoso/ identifies the location from where a Web page can be retrieved and urn:isbn:3-540-24328-3 identifies a book using its ISBN. Unicode provides a unique number for every character, independent of the underlying platform, program, or language.

Figure 1.

Semantic Web layered architecture (Berners-Lee et al., 2001)

Directly above URI and Unicode we find the syntactic interoperability layer in the form of XML, which in turn underlies RDF and RDFS. Web ontology languages are built on top of RDF and RDFS. The last three layers are logic, proof, and trust, of which the top one has not been significantly explored yet. Some of the layers rely on the digital signature component to ensure security.

Next we briefly describe the most basic layers (XML, RDF, and RDFS). While the notions presented have been simplified, they give a reasonable conceptualization of some of the simplest components of the SW.

Key Terms in this Chapter

XML: Extensible markup language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). XML is a standard for data interchanged on the Web, allowing for the structuring of data but without meaning.

RDF: Resource description framework is a family of World Wide Web Consortium (W3C) specifications originally designed as a metadata model using XML but which has come to be used as a general method of modeling knowledge, through a variety of syntax formats.

OWL: A markup language for publishing and sharing data using ontologies on the Internet. OWL is a vocabulary extension of the RDF and is derived from the DAML+OIL Web Ontology Language.

Semantic Web: SW provides a common framework that allows data to be shared and reused across applications, enterprises, and community boundaries. It is a collaborative effort led by W3C with the participation of a large number of researchers and industrial partners.

Ontology: A description of concepts and relationships that can be used by people or software agents who want to share information within a domain.

Linked USDL: It is a master schema constructed using semantic web technologies and linked data principles. It is used to describe typical services such as the ones found in healthcare, education, and cloud computing environments.